It is well known that accurate reception of wireless communications can be affected by ambient noise that is present in the frequency band or “channel” in which the communication is transmitted. In addition, signal reception can be further impeded by interference that arises from various sources, including unshielded electronic devices, electric motors, and other wireless transmitters. The latter category can arise from inadvertent transmissions, such as another system innocently transmitting at the same frequency, or at a lower frequency that gives rise to an interfering harmonic. Interference can also be malicious, for example in a combat environment where hostile forces intentionally attempt to jam communications. The degree of interference, especially of malicious interference, can change frequently for each frequency channel.
Often, wireless communication systems are able to adjust their transmit power so as to overcome ambient noise and interference. Furthermore, many wireless communication systems, including cognitive radio systems, are able to transmit and receive messages over more than one frequency band, referred to herein as a communication channel, such that it can be important to select the available communication channel that has the lowest noise and interference. Accordingly, rapid and accurate characterization of the noise and interference that is present within a communication channel, and a corresponding estimation of the transmission power that is necessary to avoid undue communication performance loss, can be important to successful wireless communication of signals of interest.
With reference to FIG. 1, if the noise and interference in a given channel is relatively constant, and if there is a known time period when the signal of interest is not present at the receiver, one method of providing an estimate of the performance loss in a communication channel is to couple the received noise and interference 104 with a local representation of the signal of interest, i.e. an analog “test” signal 102 that is generated by an analog test signal generator 106 and adjusted by a gain control 108 to a known amplitude. The analog test signal 102 is introduced into the wireless receiver 100 together with the received interference 104, and the receiver 100 attempts to successfully receive the test signal 102. An appropriate metric, such as a bit error rate, correlation score, etc., can then be used to assess the level of impairment in the communications channel.
It is important that the analog test signal 102 should be approximately equal in amplitude with the received interference 104, because a very strong analog test signal 102 will likely be received without errors, while a very weak analog test signal 102 will likely not be received at all. A disadvantage of the above method is that an analog loopback path including a coupler 110 and mixer 112 must be provided in the receiver hardware 100. Furthermore, the test signal 102 that emerges from the gain control 108 must be precisely and frequently calibrated.
What is needed, therefore, is a system and method of rapidly and accurately characterizing noise and interference that are present in a receiver channel without requiring precise and frequent calibration of test apparatus.